1. Field of the Invention
This invention relates generally to a transducer for measuring physical parameters, and more specifically to a variable reluctance transducer especially adapted to determine the load carried by a structural member by measuring the strain or deflection therein.
2. Description of the Prior Art
For safety and regulatory reasons, a need exists to determine the load being carried by structural members such as components of aircraft landing gear and axles of trucks, trailers and other vehicles. From these load measurements, it is possible to compute the weight being carried by aircraft, trucks and other vehicles to insure that they are not loaded beyond their capacity to safely carry loads or beyond the maximum weight limits set by various government bodies. Also, from this information the speed and other flight parameters of airplanes can be optimized to achieve the most fuel efficient flight conditions. Furthermore, from knowledge of the loads being carried by aircraft landing gear, it is possible to calculate the center of gravity of aircraft to make certain that it is not too far rearward to cause the aircraft to pivot rearwardly about its rear landing gear or too far forward to impair the ability of the aircraft to take off safely. Monitoring the load carried by aircraft landing gear also permits a determination to be made whether an impact caused by a hard landing was of sufficient magnitude to either damage the landing gear or require that an inspection of the landing gear be made to determine if any structural damage has occurred. Moreover, ascertaining the loads carried by aircraft landing gear members and vehicle axles permits the determination of the weight carried by each tire to thereby insure that the tires are properly inflated and are not loaded beyond safe limits.
The currently employed manner in determining the weight carried by structural beams such as aircraft landing gear members or truck axles is by the use of a strain gage type of transducer. U.S. Pat. Nos. 3,390,575 to Hukle and 3,464,259 to Farr disclose flexural beam type strain gage transducers which are mounted on a pair of lugs located along the length of and extending transversely outwardly from the outer surface of a landing gear. The lugs have flat surfaces disposed substantially coplanar to each other on which the transducer is mounted. The transducers disclosed in these two patents theoretically measure the relative movement between the two mounting lugs to thereby determine the load being transmitted through the axle.
In another type of strain gage transducer system disclosed in U.S. Pat. No. 3,521,484 to Dybvad et al, two cylindrically shaped disks are securely attached to the inside diameter of a hollow structural member, such as an aircraft landing gear axle. Each disk includes a central hub portion for carrying one end portion of an elongate sensing beam to which strain gages have been attached. The sensing beam deforms in response to the flexure of the loaded axle. As the sensing beam deforms, the physical dimensions of the strain gage wire changes thereby causing a change in the electrical resistance of the wire. The strain gages are wired together to form the arms of a Wheatstone bridge circuit producing an output signal theoretically proportional to the shear forces acting on the axle.
Strain gage type transducers, however, have drawbacks which reduce the desirability of their use, especially in such harsh environments as are associated with aircraft landing gears and truck axles. The components of strain gage transducers, including the very fine wires composing strain sensitive elements of the gage itself, are susceptible to damage from handling during shipment or storage and during the installation of the transducer. Even when strain gage type transducers are installed without being damaged, they are susceptible to failure from the shock loads and impacts normally imposed on aircraft landing gears and vehicle axles.
Another disadvantage of strain gage type transducers is that the resistance in the strain sensitive elements tends to change with the surrounding temperature thereby generating erroneous electrical output signals unless a system is provided to compensate for the changes in temperature. Moreover, the particular compensating device utilized must be capable of nulling out the changes in temperature while also ensuring that the device itself is not affected by loads acting in directions other than the direction of the load being measured. Consequently, rather sophisticated temperature compensating components are required.
If the particular load or strain being measured is large, the strain gage must be mounted on a separate mounting beam or other member which in turn is secured to the particular structural member being monitored at various mounting points. As a consequence, the strain gage mounting member itself may affect the load on the structural member being measured and thereby produce an erroneous determination of the load being carried by the structural member. Also, the manner of connecting the strain gage mounting member to the mounting points of the member being measured may cause an undue initial load to be placed on such connection points also resulting in an incorrect load measurement.
Another drawback of strain gage type transducers stems from the fact that the electrical output voltage produced by these transducers is quite small, usually on the order of a few millivolts, and has significant source resistance. Thus, the accuracy of the strain gage system may be severely reduced by a low impedance electrical leakage caused by moisture either within the transducer or in the wiring to the transducer.
It is therefore a primary goal of the present invention to provide a variable reluctance transducer which is free of the above described deficiencies of strain gage type transducers to thereby be capable of accurately measuring the loads on structural components such as aircraft landing gear members. Variable reluctance transducers have in the past been used to measure pressures. One example of this type of transducer is disclosed by Tavis U.S. Pat. No. 3,118,121, wherein two spaced apart magnetic cores are fixedly held within a cylindrical housing. Each core is adapted to support a wire wound coil. A diaphragm plate separates the two cores and divides the housing into two individual pressure chambers. Magnetic plates are welded to each side of the diaphragm plate to lie closely adjacent each core. Ports are provided in the housing to enable fluid under pressure to enter each chamber, with the pressure differential existing across the diaphragm causing it to deflect toward the chamber subjected to the lower pressure. As the differential pressure responsive diaphragm is deflected, the magnetic plates move relative to their respective cores thereby altering the magnetic reluctance of the magnetic path extending through the core.
A variable reluctance transducer designed to measure the pressure of the fluids in a manner similar to the above described Tavis '121 patent is disclosed by Tavis U.S. Pat. No. 3,562,687. In this particular transducer, each core is constructed from a circular, center pole plate from which a plurality of slender, spaced apart legs extend radially outwardly. Each of the legs is U-shaped to thereby define a segmented, annular channel for receiving a coil therein. An enlarged, arcuate tab extends laterally outwardly from the end of each leg opposite the center pole plate to thereby define a segment annularly shaped outer pole face which is substantially coplanar with the center pole plate.